1. Field of the Invention
The present invention relates to a light receiving element and a solar cell including the light receiving element, and suitably to a light receiving element including a quantum dot layer and a solar cell including the light receiving element.
2. Description of the Background Art
Various researches and developments of solar cells representing one example of devices including light receiving elements have been carried out for the purpose of enhancement of photoelectric conversion efficiency by making use of light of a wider wavelength range. For example, a solar cell in which a superlattice miniband is formed between a valence band and a conduction band of a base material by making use of quantum dot technology and electrons are photoexcited in two steps with the superlattice miniband being interposed, so that light of a long wavelength can be made use of has been proposed (for example, Japanese Patent Laying-Open No. 2006-114815, Japanese National Patent Publication No. 2010-509772, PHYSICAL REVIEW LETTERS, Vol. 97, page 247701, 2006, or PHYSICAL REVIEW B, Vol. 82, page 195321, 2010).
Such a solar cell containing quantum dots has a quantum dot layer inserted, which contains quantum dots in a base semiconductor forming an i-type semiconductor layer of a compound solar cell. By inserting a quantum dot layer in a base semiconductor, electronic bond between quantum dot layers is formed and hence a superlattice miniband is formed. With two-step photoexcitation through the superlattice miniband, light in a wavelength region which has not been made use of can be absorbed (absorption of photons lower in energy than a band gap of a base semiconductor material) so that a photocurrent can be increased. Carriers generated in quantum dots migrate through the superlattice miniband and are extracted to the outside by photoexcitation. Carriers absorbing light of a long wavelength and excited to a quantum level of a quantum structure, however, are recombined before they are extracted to an electrode.
Then, extension of a lifetime of carriers by obtaining a band structure of a quantum structure of a type II structure has been studied (Japanese Patent Laying-Open No. 2006-114815 and APPLIED PHYSICS LETTERS, Vol. 93, page 033107, 2008). By obtaining the band structure of the type II structure, generated electrons and holes are spatially separated and thus a lifetime of carriers can be extended. For example, it has been known that the type II structure can be obtained by employing InAs for quantum dots and employing GaAs1-xSbx for a barrier layer, and spatial separation of carriers is enhanced and a lifetime of carriers tends to extend by increasing a ratio of x.